Re: [stage1][PATCH] Lower VEC_COND_EXPR into internal functions.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Apr 6, 2020 at 11:18 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Martin Liška <mliska@suse.cz> writes:
> > Hello.
> >
> > This is second attempt to get rid of tcc_comparison GENERIC trees
> > to be used as the first argument of VEC_COND_EXPR.
> >
> > The patch attempts achieves that in the following steps:
> > 1) veclower pass expands all tcc_comparison expression into a SSA_NAME
> > 2) since that tcc_comparsion can't be used as the first argument of VEC_COND_EXPR
> >     (done in GIMPLE verifier)
> > 3) I exposed new internal functions with:
> > DEF_INTERNAL_OPTAB_FN (VCOND, 0, vcond, vec_cond)
> > DEF_INTERNAL_OPTAB_FN (VCONDU, 0, vcondu, vec_condu)
> > DEF_INTERNAL_OPTAB_FN (VCONDEQ, 0, vcondeq, vec_condeq)
> > DEF_INTERNAL_OPTAB_FN (VCOND_MASK, 0, vcond_mask, vec_cond_mask)
> >
> > 4) logic of expand_vec_cond_expr is moved into the new pass_gimple_isel pass
> > 5) the pass expands VEC_COND_EXPR into one of the internal functions defined in 3)
> > 6) moreover, I've added a new logic that prefers expand_vec_cmp_expr_p when
> >     a SSA_NAME is being used in multiple (2+) VEC_COND_EXPR statements
> >
> > Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> > Moreover, I run SPEC2006 and SPEC2017 benchmarks on znver1, znver2 and skylake
> > target and I don't see any reasonable change.
> >
> > Achieved benefits of the patch:
> > - removal of a GENERIC expression being used in GIMPLE statements
> > - extraction into SSA_NAMEs can enable proper tree optimizer (FRE, DOM, PRE)
> > - possibility to expand smarter based on number of uses (expand_vec_cmp_expr_p)
> >
> > Future plans:
> > - tcc_comparison removal just during gimplification
> > - removal of a code where these expressions are handled for VEC_COND_EXPR
> > - do the similar thing for COND_EXPR?
> >
> > The task was guided by Richi (Biener) and I bet he can help with both further questions
> > and reasoning.
>
> Thanks for doing this.  It definitely seems more friendly than the
> four-operand version to targets where separate comparisons are the norm.
>
> Just a couple of comments about the implementation:
>
> > diff --git a/gcc/passes.def b/gcc/passes.def
> > index 2bf2cb78fc5..d654e5ee9fe 100644
> > --- a/gcc/passes.def
> > +++ b/gcc/passes.def
> > @@ -397,6 +397,7 @@ along with GCC; see the file COPYING3.  If not see
> >    NEXT_PASS (pass_cleanup_eh);
> >    NEXT_PASS (pass_lower_resx);
> >    NEXT_PASS (pass_nrv);
> > +  NEXT_PASS (pass_gimple_isel);
> >    NEXT_PASS (pass_cleanup_cfg_post_optimizing);
> >    NEXT_PASS (pass_warn_function_noreturn);
> >    NEXT_PASS (pass_gen_hsail);
>
> What was the reason for making this a separate pass, rather than doing
> it as part of veclower?  If we do them separately, then it's harder for
> veclower to know which VEC_COND_EXPRs it needs to open-code.  (OK, so
> that's a general problem between veclower and expand already, but it
> seems like the new approach could help to move away from that by
> doing the instruction selection directly in veclower.)
>
> > +/* Expand all VEC_COND_EXPR gimple assignments into calls to internal
> > +   function based on type of selected expansion.  */
> > +
> > +static gimple *
> > +gimple_expand_vec_cond_expr (gimple_stmt_iterator *gsi)
> > +{
> > +  tree lhs, op0a = NULL_TREE, op0b = NULL_TREE;
> > +  enum tree_code code;
> > +  enum tree_code tcode;
> > +  machine_mode cmp_op_mode;
> > +  bool unsignedp;
> > +  enum insn_code icode;
> > +  imm_use_iterator imm_iter;
> > +
> > +  /* Only consider code == GIMPLE_ASSIGN.  */
> > +  gassign *stmt = dyn_cast<gassign *> (gsi_stmt (*gsi));
> > +  if (!stmt)
> > +    return NULL;
> > +
> > +  code = gimple_assign_rhs_code (stmt);
> > +  if (code != VEC_COND_EXPR)
> > +    return NULL;
> > +
> > +  tree op0 = gimple_assign_rhs1 (stmt);
> > +  tree op1 = gimple_assign_rhs2 (stmt);
> > +  tree op2 = gimple_assign_rhs3 (stmt);
> > +  lhs = gimple_assign_lhs (stmt);
> > +  machine_mode mode = TYPE_MODE (TREE_TYPE (lhs));
> > +
> > +  gcc_assert (!COMPARISON_CLASS_P (op0));
> > +  if (TREE_CODE (op0) == SSA_NAME)
> > +    {
> > +      unsigned int used_vec_cond_exprs = 0;
> > +      gimple *use_stmt;
> > +      FOR_EACH_IMM_USE_STMT (use_stmt, imm_iter, op0)
> > +     {
> > +       gassign *assign = dyn_cast<gassign *> (use_stmt);
> > +       if (assign != NULL && gimple_assign_rhs_code (assign) == VEC_COND_EXPR
> > +           && gimple_assign_rhs1 (assign) == op0)
> > +         used_vec_cond_exprs++;
> > +     }
>
> This looks like it's quadratic in the worst case.  Could we check
> this in a different way?

We could remember a SSA names cond-expr-uses and thus only compute it
once.

> > +
> > +      gassign *def_stmt = dyn_cast<gassign *> (SSA_NAME_DEF_STMT (op0));
> > +      if (def_stmt)
> > +     {
> > +       tcode = gimple_assign_rhs_code (def_stmt);
> > +       op0a = gimple_assign_rhs1 (def_stmt);
> > +       op0b = gimple_assign_rhs2 (def_stmt);
> > +
> > +       tree op0a_type = TREE_TYPE (op0a);
> > +       if (used_vec_cond_exprs >= 2
>
> It would be good if targets were able to provide only vcond_mask.
> In that case I guess we should go this path if the later one would fail.
>
> > +           && (get_vcond_mask_icode (mode, TYPE_MODE (op0a_type))
> > +               != CODE_FOR_nothing)
> > +           && expand_vec_cmp_expr_p (op0a_type, TREE_TYPE (lhs), tcode))
> > +         {
> > +           /* Keep the SSA name and use vcond_mask.  */
> > +           tcode = TREE_CODE (op0);
> > +         }
> > +     }
> > +      else
> > +     tcode = TREE_CODE (op0);
> > +    }
> > +  else
> > +    tcode = TREE_CODE (op0);
>
> Might be easier to follow if tcode is TREE_CODE (op0) by default and
> only gets changed when we want to fold in the comparison.
>
> Thanks,
> Richard